Roy Christoffersen

Redox-driven exsolution of iron-titanium oxides in magnetite in Miller Range (MIL) 03346 nakhlite: Evidence for post crystallization oxidation in the nakhlite cumulate pile?

Abstract The Miller Range (MIL) 03346 nakhlite contains ~20% mesostasis, which contains skeletal titanomagnetite. The titanomagnetite contains trellis-type {111} lamellae of ilmenite similar to those found in terrestrial titanomagnetites that have experienced subsolidus redox reactions during cooling of their host rocks. We have characterized the MIL 03346 titanomagnetite-ilmenite intergrowths by a combination of focused ion beam (FIB), energy-dispersive spectroscopy (EDX), and high-resolution transmission electron microscopy (TEM). The resulting structural and chemical analyses have been combined with temperature and fO2 data from previous studies of MIL 03346, as well as previous work on two-oxide thermobarometry of nakhlites. Our calculations show that as MIL 03346 and other nakhlites cooled below 800 °C, they recorded increasingly reducing conditions, such that the lowest temperatures calculated correspond to fO2 conditions as low as 4 log fO2 units below the FMQ buffer. However, the MIL 03346 lamellae must have formed by oxidation and thus record a very late stage lowtemperature (<350 °C) oxidation event. When considered together with previous studies of MIL 03346 and nakhlites in general, the overall cooling history could be explained by early oxidation followed by intermediate stage reduction caused by S2 loss by degassing, followed by late loss of C1 by degassing.

Transmission Electron Microscopy Studies of Carbonaceous Chondrites which Experienced Experimentally Simulated Space Weathering Effects

Introduction: Mineral grains on the surfaces of airless bodies such as the Moon and asteroids are continually modified due to their exposure to interplanetary space. Radiation processing via solar energetic ions from the solar wind and micrometeorite impacts collectively alter the microstructure, chemical composition, and optical properties of soils grains on airless planetary surfaces [1]. This phenomenon is known as space weathering, and understanding the nanoscale effects that result from this processing is essential for interpreting spectroscopic data collected by remote sensing spacecraft.

TEM Analyses of Itokawa Regolith Grains and Lunar Soil Grains to Directly Determine Space Weathering Rates on Airless Bodies

Samples returned from the moon and Asteroid Itokawa by NASAs Apollo Missions and JAXAs Hayabusa Mission, respectively, provide a unique record of their interaction with the space environment. Space weathering effects result from micrometeorite impact activity and interactions with the solar wind. While the effects of solar wind interactions, ion implantation and solar flare particle track accumulation, have been studied extensively, the rate at which these effects accumulate in samples on airless bodies has not been conclusively determined. Results of numerical modeling and experimental simulations do not converge with observations from natural samples. We measured track densities and rim thicknesses of three olivine grains from Itokawa and multiple olivine and anorthite grains from lunar soils of varying exposure ages. Samples were prepared for analysis using a Leica EM UC6 ultramicrotome and an FEI Quanta 3D dual beam focused ion beam scanning electron microscope (FIB-SEM). Transmission electron microscope (TEM) analyses were performed on the JEOL 2500SE 200kV field emission STEM. The solar wind damaged rims on lunar anorthite grains are amorphous, lack inclusions, and are compositionally similar to the host grain. The rim width increases as a smooth function of exposure age until it levels off at approximately 180 nm after approximately 20 My (Fig. 1). While solar wind ion damage can only accumulate while the grain is in a direct line of sight to the Sun, solar flare particles can penetrate to mm-depths. To assess whether the track density accurately predicts surface exposure, we measured the rim width and track density in olivine and anorthite from the surface of rock 64455, which was never buried and has a surface exposure age of 2 My based on isotopic measurements. The rim width from 64455 (60-70nm) plots within error of the well-defined trend for solar wind amorphized rims in Fig. 1. Measured solar flare track densities are accurately reflecting the surface exposure of the grains. Track densities correlate with the amorphous rim thicknesses. While the space-weathered rims of anorthite grains are amorphous, the space-weathered rims on both Itokawa and lunar olivine grains show solar wind damaged rims that are not amorphous. Instead, the rims are nanocrystalline with high dislocation densities and sparse inclusions of nanophase Fe metal. The rim thicknesses on the olivine grains also correlate with track density. The Itokawa olivine grains have track densities that indicate surface exposures of approximately 10(exp 5) years. Longer exposures (up to approximately 10(exp 7) years) do not amorphize the rims, as evidenced by lunar soil olivines with high track densities (approximately 10(exp 11) cm(exp -2)). From the combined data, shown in Fig. 1, it is clear that olivine is damaged (but not amorphized) more rapidly by the solar wind compared to anorthite. The olivine damaged rim forms quickly (in approximately 10(exp 6) y) and saturates at approximately 120nm with longer exposure time. The anorthite damaged rims form more slowly, amorphize, and grow thicker than the olivine rims. This is in agreement with numerical modeling data which predicts that solar wind damaged rims on anorthite will be thicker than olivine. However, the models predict that both olivine and anorthite rims will amorphize and reach equilibrium widths in less than 10(exp 3) y, in contrast to what is observed for natural samples. Laboratory irradiation experiments, which show rapid formation of fully amorphous and blistered surfaces from simulated solar wind exposures are also in contrast to observations of natural samples. These results suggest that there is a flux dependence on the type and extent of irradiation damage that develops in olivine. This flux dependence suggests that great caution be used in extrapolating between high-flux laboratory experiments and the natural case, as demonstrated by. We constrain the space weathering rate through analysis of returned samples. Provided that the track densities and the solar wind damaged rim widths exhibited by the Itokawa grains are typical of the fine-grained regions of Itokawa, then the space weathering rate is on the order of 10(exp 5) y. Space weathering effects in lunar soils saturate within a few My of exposure while those in Itokawa regolith grains formed in approximately 10(exp 5) y. Olivine and anorthite respond differently to solar wind irradiation. The space weathering effects in olivine are particularly difficult to reconcile with laboratory irradiation studies and numerical models. Additional measurements, experiments, and modeling are required to resolve the discrepancies among the observations and calculations involving solar wind amorphization of different minerals on airless bodies.